Optical scattering techniques are in wide use for inspecting highly polished surfaces, such as those of lenses and silicon wafers as employed as starting materials in semiconductor manufacture. These techniques involve directing a monochromatic beam of incident light onto the surface. Most of the beam is “specularly” reflected, that is, is reflected at an angle of reflection equal to the angle of incidence; however, a small fraction of the beam is “scattered” into other directions. The amount of light scattered is generally representative of the roughness of the surface and the presence of particulates thereon, or defects therein.
Optical scattering techniques provide a powerful tool for process monitoring in manufacturing environments because of their non-contact nature and relative ease of use. For example, optical scattering techniques are often employed to detect particulate contamination of silicon wafers on fabrication lines. The requirement that particles smaller than the minimum dimension of the features to be fabricated on the wafer, which can be reliably detected, places strict demands on the sensitivity of an instrument to those particles. One important issue that limits the sensitivity of such an instrument to particulates and other small defects is the noise associated with the scattering signal due to background sources including surface microroughness.
The full strength of the optical scattering technique lies in its ability to diagnose deviations from ideal conditions. For example, optical scattering from smooth surfaces, such as mirrors, transparent optics, and silicon wafers can yield information about the condition of those surfaces. Surface microroughness, particulate contamination, and subsurface defects result from different adverse conditions in the manufacturing environment; distinguishing between these sources of defects can result in improvements in the ability to identify and correct the sources of such conditions.
Current scanning surface inspection systems employ optical scattering techniques to detect microroughness, particles, and defects in silicon wafers. Light is focused onto the surface of the wafer, and optics collect light that is scattered by the surface and image it onto a sensitive detector. Some degree of noise, whether it is from microroughness of the surface of the sample or from some other source, is always present in the scattered radiation signal. This noise has a tendency to hide detection of the smallest particles and defects. Reduction of this noise improves the systems ability to detect smaller particulates and defects.
U.S. Pat. No. 6,034,776 (hereinafter, the '776 patent) discloses a microroughness-blind optical scanner that focuses p-polarized light onto a surface of a sample. Scattered light is collected through independently rotatable polarizers by one or more collection systems uniformly distributed into several regions over a hemispherical shell centered over the sample. In each separate collector region, a unique linear polarizer is adjusted to a specific angle that nulls or minimizes the microroughness-induced scatter in that region for a given surface to be inspected.
U.S. patent application Ser. No. 11/110,383, filed Apr. 20, 2005, the contents of which are incorporated herein by reference, discloses an inspecting system including an illumination subsystem configured to direct light to the specimen at an oblique angle of incidence. The light is polarized in a plane that is substantially parallel to the plane of incidence. The system also includes a detection subsystem configured to detect light scattered from the specimen. The detected light is polarized in a plane that is substantially parallel to the plane of scattering. This prior art teaches a “pizza-pie-polarizer” 310 as shown in FIG. 3A. The polarizer 310 is a segmented polarizer formed of multiple sections 312 of linear polarizers butted against each other, each having a different orientation 314 for pass axis.
U.S. Pat. No. 7,221,501 also teaches a radial transverse electric polarizer device including a first layer of material having a first refractive index, a second layer of material having a second refractive index, and a plurality of elongated elements azimuthally and periodically spaced apart, and disposed between the first layer and the second layer. The plurality of elongated elements interact with electromagnetic waves of radiation to transmit transverse electric polarization of electromagnetic waves of radiation.
U.S. Pat. No. 7,436,505 discloses a system for determining a configuration for a light scattering inspection system. The optimal configuration may be further refined by determining the effect that polarizing filter element(s) placed in the path(s) of the detected light will have on the sensitivity of the inspection system. In this manner, in some embodiments, the configuration also includes parameter(s) of one or more linear polarizing filters positioned in the scattering hemisphere. In this manner, the determined optimal configuration may be realized by positioning linear polarizing filters in the opening(s) of an aperture plate. The polarizing filter includes a plurality of segments, each of which is a linear polarizing filter arranged azimuthally in the aperture plate openings, which may be commonly referred to as a “pizza-pie” polarizer. The linear polarizing filters may also be disposed in any location with respect to the openings such that light that passes through the openings also passes through the linear polarizing filters. In other words, the linear polarizing filters do not have to be disposed in the openings, but may be disposed upstream or downstream of the aperture.